A lemma in the integers from calculus

Suppose that M and N are natural numbers, such that N>M-1.
Prove that N≥M

The problem above is a rather minor lemma that I obtained while proving the ratio test from calculus. I was able to successfully prove the ratio test itself, but I took this lemma for granted, which I am now trying to prove.

I expected that since this lemma was rather simple, it would be easy enough to prove, but I can't seem to catch it.

Here's a proof that there are no integers between 0 and 1: Suppose that s is an integer such that 0 < s < 1. Let S = {positive integers n : 0 < n < 1}. Then S is not empty because s [itex] \in[/itex] S. By the well-ordering principle, S contains a least element m. Thus, 0 < m < 1. Multiplying by m, we get 0 < [itex]m^2[/itex] < m. Thus [itex]m^2 \in [/itex] S and [itex]m^2[/itex] < m, a contradiction. Therefore no such s exists, so there are no integers between 0 and 1.

Whether it requires proof that there are no integers between 0 and 1 depends on the context. Most people would accept it without proof. If you are writing this up for a class, then you could check with your professor. In most classes, even ones that start by proving of all the basic properties of numbers, by the time you get to infinite series, then you have probably moved on past minor points such as this. So mathsman's argument would suffice.

However, since you were asking about the proof of that lemma, a careful proof would involve the Well Ordering Principle and it would follow Petek's argument.